Recently, interest in energy storing technologies is gradually increasing. As application fields expand to energies for mobile phones, camcorders and notebook PCs, and further, electric vehicles, efforts for research and development on electrochemical devices are gradually materializing. Electrochemical device is a field gathering the most attention in this aspect, and especially, development on chargeable and dischargeable secondary batteries is becoming the focus of interest. Recently, in developing such batteries, research and development are underway to design new electrodes and batteries in order to improve capacity density and specific energy.
Of the secondary batteries that are currently being applied, lithium secondary batteries developed in the early 1990s are coming into the spotlight due to advantages of higher operating voltage and much greater energy density compared to conventional batteries such as N—MH, Ni—Cd, lead-sulfate batteries and the like that use aqueous solution electrolyte. However, these lithium ion batteries have safety problems such as ignition, explosion and the like, caused by using organic electrolyte, and disadvantages of difficulty in manufacturing.
Evaluating safety and securing safety of the aforementioned batteries is very important. The most important consideration is that the batteries should not inflict injury to a user when they malfunction, and for this purpose, ignition, smoke generation and the like inside the batteries are being strictly regulated by battery safety standards. Therefore, in order to resolve the safety problems, numerous resolving methods are being presented.
For example, lithium ion batteries and lithium ion polymer batteries generally use polyolefin series separators to prevent short circuit of a positive electrode and a negative electrode. However, a polyolefin series separator has disadvantages of the characteristic of the material of the separator, for example, the characteristic of the polyolefin series generally melting at 200□ or below, and the processing characteristic, for example, heat-shrinking according to its original size at a high temperature due to the characteristic of going through a stretching process for adjusting the pore size and porosity. Therefore, in the case where the temperature of a battery rises due to an internal/external stimulus, there is high possibility of a positive electrode and a negative electrode being shorted from each other due to the shrinking or melting and the like of the separator, and by emission and the like of electrical energy due to the aforementioned, the battery will show high risk of explosion and the like.
In order to compensate the aforementioned, a composite separator, in which an inorganic particle coating layer is formed on a polyolefin series material, was proposed, but such a composite separator has weak adhesion strength to electrodes, and has a relatively thick thickness that does not live up to the recent tendency of separators becoming thinner, and thus there needs to be improved.
Recently, an electrode-separator integrated electrode assembly is being proposed, where an inorganic coating layer is formed on an electrode active material layer to serve as a conventional separator. However, the inorganic coating layer formed in the electrode active material layer as aforementioned is not coated on a material such as a polyolefin film, and thus has a problem of having low mechanical strength and easily generating cracks, and therefore, there was a problem of difficulty in securing safety in an electrochemical device using such an electrode-separator integrated electrode assembly.